1. Field of the Invention
This invention pertains generally to a device for determining the draft of a surface vessel or the depth of a submerged vehicle and more particularly to the accurate measurement of these quantities relative to mean sea level.
2. Description of the Related Prior Art
A surface vehicles draft is traditionally measured by observing the position of the waterline on hull draft markings. However, it is not feasible to accurately estimate draft in this way when there is wave action. Also, this measurement cannot be accomplished while the vehicle is underway. Another method for measuring draft of a vehicle is by estimation through computation by accounting for the loading of fuel, water and cargo on the vehicle. This is not accurate and does not compensate for squat and lift effects when the vehicle is moving. Squat and lift cause changes in a vehicles draft as a function of vehicle speed, water current and water depth and cannot be accurately computed.
When in close proximity to a shore having a kinematic Global Positioning System (GPS) which measures vehicle position with respect to a GPS reference point, the draft is computed by adjusting the GPS reference point for the tide level at the vehicle's position. Accurate tide level requires placement of a tide sensor in the proximity of the vehicle. Because of the requirements for off-board sensors, this approach is not generally practical for a transiting vehicle.
In submerged vehicles, depth is traditionally measured by a pressure sensor. With this method of measurement errors occur due to head pressure effects and fluid flow over a moving vehicle; pressure effects due to wave action when the vehicle is near the surface; vertical current components in the water through which the vehicle is moving; and changes in the water density due to temperature or salinity changes.
An acoustic ranging device can be used to determine the travel time of an acoustic pulse, and thus the distance, from the vehicle to the water's surface. For near surface applications the measured range signal is contaminated by surface wave action. Measurement of vertical displacement can also be achieved for a subsurface vehicle using a pressure depth sensor, but the same errors are experienced as discussed above for measuring vehicle depth.
The measurement of vertical displacement for a surface or subsurface vehicle is traditionally accomplished using a heave sensor. A typical heave sensor uses accelerometers to measure the vertical acceleration of the vehicle in order to generate heave. The state of the art heave for sensors, is shown in U.S. Pat. No. 4,986,121; APPARATUS FOR MEASURING THE VERTICAL MOTION OF A FLOATING PLATFORM; Luscombe; Jan. 22, 1991; U.S. Pat. No. 4,697,253; SONAR HEAVE COMPENSATION SYSTEM; Lind et al.; Sep. 29, 1987; and U.S. Pat. No. 4,104,608; HEAVEMETER; Melling et al.; Aug. 1, 1978. These devices compute heave by double integrating measured vertical acceleration of the vehicle to obtain vertical displacement (heave). Due to the double integration there are two unknown constants of integration and the average value of this signal is thus meaningless and is removed in the heave sensor using a high pass filter. Due to the employment of the high pass filter in a heave sensor, the output cannot be correctly interpreted as vertical displacement after abrupt changes in the vehicle's mean path. The result is a signal that is the heave of the vehicle about an unknown mean path; a heave of zero indicates no vertical displacement from this path. This method of measuring vertical displacement is effective in cases where the mean path of a vehicle does not change or changes very slowly with time. With a heave sensor, knowledge of the vertical position of a vehicle with respect to an earth reference frame requires addition of the heave to draft or depth.
Due to the employment of a high pass filter in the heave sensor, however, the heave signal cannot be used as the vertical displacement of the vehicle after abrupt changes in the vehicle's mean path. FIG. 1 shows the unit step response of a typical heave system high pass filter, denoted G(s). In FIG. 1, the the sensor is moved 1 unit upward at time zero and left in that position, representing the new mean path. It is evident in FIG. 1 that this change in mean path results in a long transient (several minutes) that represents an error in vertical displacement from the new mean path. Changes in a surface vehicles mean path, i.e., draft, may occur due to a change in loading or due to squat and lift effects. Changes of a submerged vehicle's mean path, i.e., depth, may occur due to a change in the command depth.